The present invention relates to a sheet feeding apparatus for sequentially separating sheets on a stacker into a single sheet and feeding the sheet one by one to a processing station for reading images or printing. The present invention also relates to a method of detecting a double feed of a plurality of sheets in a sheet feeding process.
A conventional sheet feeding apparatus sequentially feeds sheets stacked on a stacker to a processing station such as a printer, copier, or scanner. It is necessary to accurately separate the sheets into a single sheet and to detect a double feed of the sheets before the sheet reaches the processing station, so that it is possible to stop processing or discard processing data such as reading information. In the process of separating the sheets stacked on a stacker into a single sheet and feeding the sheet one by one to the processing station, if the double feed of the sheets occurs, an incorrect process is applied to the sheets at the processing station.
A conventional method of detecting the double feed of the sheets includes an ultrasonic sensor, a photo-sensor, or the like for detecting attenuation in an ultrasonic wave or an intensity of light passing through the sheet, thereby determining whether there is a single sheet.
Japanese Patent Publication (Kokai) No. 10-257595 discloses an ultrasonic sensor for detecting a sheet. The ultrasonic sensor includes a piezoelectric oscillation plate such as piezoelectric ceramic at a wave transmission side. A pulse voltage with a predetermined frequency is applied to the piezoelectric oscillation plate to generate oscillation, thereby transmitting ultrasonic waves. A similar oscillation plate is provided at a wave reception side for receiving ultrasonic waves and converting to an electrical signal. Electric energy applied to the piezoelectric oscillation plate (wave transmission element) at the wave transmission side is compared with electric energy generated at the piezoelectric oscillation plate (wave reception element) at the wave reception side, thereby determining a single sheet or several sheets.
When the double feed is detected with such an ultrasonic sensor, it is necessary to accurately measure ultrasonic energy (electric energy output from the wave reception element) attenuated through the sheet between the wave transmission element and the wave reception element. U.S. Pat. No. 6,212,130 discloses a conventional structure in which a wave transmission element and a wave reception element are arranged opposite to each other with a predetermined angle relative to a surface of a sheet. With this structure, it is possible to prevent ultrasonic waves transmitted from the wave transmission element from reflecting at the sheet surface and interfering.
Japanese Utility Model (Kokoku) No. 06-49567 proposes a structure in which a wave transmission element and a wave reception element are arranged opposite to each other between a downstream roller and an upstream roller arranged with a predetermined distance in between, thereby making it possible to detect the double feed while a sheet is in a stable condition. More specifically, with such a structure, the double feed is detected while the downstream and upstream rollers nip the sheet in a straight position during transportation. Accordingly, it is possible to accurately detect the double feed since a leading end or a trailing end of the sheet is not curved or does not oscillate vertically. When ultrasonic waves or an intensity of light transmitting the sheet moving at a predetermined speed is measured to determine a difference between a single sheet and several sheets, it is necessary to reduce a variation in a posture of the sheet. It is also necessary to measure and smooth data over a predetermined length (region) of the sheet.
In order to detect the double feed of the sheets using the double feed detection device such as an ultrasonic wave sensor described above, it is necessary to hold the sheet between a pair of rollers at front and rear sides of the sheet, so that the sheet travels between the transmission element and the reception element in a constant posture, such as the case proposed in Japanese Utility Model (Kokoku) No. 06-49567. It is also necessary to smooth measured data continuously detected over a specific region of the sheet to determine the double feed.
In the conventional structure disclosed in Japanese Utility Model (Kokoku) No. 06-49567, the rollers are arranged at front and rear sides of the sheet in the transport direction with a distance in between shorter than a length of the minimum size sheet, and the double feed is detected using detection data measured over a measurement range (region) smaller than the distance. When sheets in a wide size range from small to large are transported, if the measurement range is set according to a length of the minimum size sheet in the transport direction, it is possible to detect a part of a leading edge and send the sheet to a sheet processing unit as a single sheet when two large size sheets are overlapped and shifted in the transport direction. When the measurement range is set according to a length of a middle or large size sheet in the transport direction, it is also possible to cause an erroneous detection as a trailing edge of a small sized sheet comes off a transport device and flaps when a detection sensor detects the double feed.
In view of the problems described above, an object of the present invention is to provide a method of detecting double feed in which it is possible to accurately determine a single sheet or several sheets when sheets with different lengths in a transport direction are transported. In particular, it is possible to accurately detect two sheets overlapped each other and shifted in the transport direction.
Another object of the present invention is to provide a sheet feeding apparatus using the method of detecting the double feed.
A further object of the present invention is to provide a sheet feeding apparatus and a method of detecting the double feed, in which it is possible to accurately detect the double feed of sheets fed from a stacker with a simple structure when sheets with sizes different from standard are transported or sheets are shifted in the transport direction.
Further objects and advantages of the invention will be apparent from the following description of the invention.